Our present invention relates to a stretch leveler for steel and other metal strip. More particularly, the stretch leveler of the invention is intended for metal strip having a thickness range of substantially 0.1 to 4 mm between a minimum thickness and a maximum thickness and a strip width range between a minimum width and a maximum width which can be 600 to 1850 mm typically.
In general, stretch levelers have in the past been provided with a multiplicity of bridles which have alternately acted as braking roll sets and traction roll sets and between which respective stretching or stretch leveling zones have been formed. As a practical matter, at least two stretching or leveling zones have been provided in such systems.
Because of the subdivision of the leveling effect into two (or more) leveling zones, planarity can be improved with respect to single zone stretch levelers since in an earlier stretching zone the strip width is elastically reduced and in a subsequent leveling zone a more uniform tension distribution can be provided across the width of the strip so that the resulting strip will have greater planarity.
In the prestretching zone the strip tension can be raised practically to the yield limit RP0.2 so that in combination with the bending effect determined by the final diameter of the tensioning drum or drums, there is a slight elastoplastic prestretch. As a consequence any deviation from planarity is partly removed as early as the prestretching zone. In such earlier systems it is theoretically also conceivable to raise the strip tension above the RP0.2 value or to the RP0.2 value in the prestretching zone.
There are stretch leveling systems known as well in which between the brake roll set and a traction roll set, a further roll pair can be provided to engage the strip. In that case, between each of those roll sets and the additional roll pair, there are formed respective stretching zones. A plastic deformation of the strip, however, appears to occur only in the region of the additional roll pair. For a satisfactory leveling action, however, the plastic stretching of the strip must be distributed between the roll sets and the additional roll pair (compare DE 39 12 676 C2).
In another system (DE 196 45 599) stretching regions are provided with a more complex roll arrangement between the brake roll set and the traction roll set.
Finally, as to the art, a stretch-bending system with three stretch bending rolls is known from DE 36 36 707 C2 in which the strip is bent alternately in opposite directions and a central stretch bend roll must be located between two other rolls which alternately are undershot and overshot by the strip.
It is the principal object of the present invention to provide a stretch leveling apparatus which is of simplified construction and which can reliably and with good and effective results impart planarity to steel and other metal strip.
Another object of the invention is to provide an apparatus which overcomes drawbacks of earlier systems and which can in a simple way ensure good planarity results for the stretch leveling of steel and other metal strip.
These objects and others which will become apparent hereinafter are attained, in accordance with the invention in an apparatus which comprises:
a brake roll set having a plurality of brake rolls around which a traveling metal workpiece strip passes for exerting a drag upon the traveling metal workpiece strip;
a traction roll set spaced from the brake roll set and having a plurality of traction rolls around which the traveling metal workpiece strip passes for exerting traction upon the traveling metal workpiece strip; and
a driven roll engaging the traveling metal workpiece strip between the brake roll set and the traction roll set and defining a first leveling stretching zone between the brake roll set and the driven roll and a second leveling stretching zone between the driven roll and the traction roll set such that each of the zones has a length which is at least 0.5 times the maximum strip width.
According to a feature of the invention lengths of the first and second stretch leveling zones are each a maximum of ten times the maximum strip width. In a preferred embodiment the length of the first zone and the length of the second zone are each one to two times the maximum strip width. The diameters of all of the rolls described should be at least 1,000 times the maximum strip width.
Preferably the last roll of the tracking set and the first of the traction set and the driven roll have concave/convex contours which are adjustable. The adjustment can be effected zonewise over the width of the strip. At least one of the zones can be associated with a linear motor which influences the strip tension distribution across the width of the strip. The bending direction of the driven roll can be opposite that of the first roll of the traction set and the residual longitudinal curvature (coil set) or transverse curvature (bowing) in the strip can be corrected by adjustment to the ratio of the degree of stretch in the two zones.
According to a feature of the invention, the transverse curvature or bowing can be measured with an in-line sensor on a real-time or on-line basis and the measurement can be used as a parameter for a closed-control circuit for correction of the curvature.
Ahead of, in, or downstream of the leveling zone the planarity of the strip can be measured on an on-line basis and the measurement used as a parameter for planarity control of the stretch stages.
The strip can be slung around the driven roller by at most 180xc2x0 and preferably at most 90xc2x0. The lengths of the first and second zones can be variable and adjusted to optimal lengths for the given strip thicknesses.
According to the invention, based upon theoretical calculations utilizing a dynamic finite element model, it has been found surprisingly that the lengths of the stretching zones mentioned above constitute an important criterium for the uniformity of the residual stress distribution across the strip width and thus the degree of planarity. The longitudinal tension stresses are constant across the width of the strip following leveling. Residual stress upon relief of the load can be zero and the strip ideally planar.